The previous system is that if you cast charm spells on a group of monsters, they will wander off. This still holds if you don't charm all members of a group. However, if your spell successfully charms all the remaining monsters, they will stay and become peaceful. This has one significant advantage: Resting in a square with a peaceful monster guarantees that you will not be attacked. I'll probably have to up to power of charm spells to make this viable, but I thought it would be a fun twist...